Valve collets, also known as valve keepers or valve locks, are small, wedge-shaped components used to secure the valve spring retainer to the valve stem. They fit into grooves on the valve stem and are compressed by the valve spring retainer to hold the spring in place. Valve collets ensure that the valve assembly remains securely in position, allowing for precise valve operation and maintaining correct valve timing. Made from durable materials like steel or hardened alloys, they are essential for the reliable performance of the engine's valve train.